Method for manufacturing synthetic acrylonitrile fibers



Uite ttes This invention relates to an improved process for the manufacture of shaped polymeric materials. In particu lar, this invention is concerned with an improved Wet spinning method for the production of fibers of acrylonitrile polymer or copolymer containing at least 85% acrylonitn'le.

The process according to this invention is generally applicable to the production of any shaped articles, for example, such as yarns, films, bristles, tubing and the like.

For conveniencel the discussion herein is directed chiefly to the wet spinning process for manufacturing excellent synthetic and polymeric fibers having high resilience and high heat resistance from polyacrylonitrile or copolymers thereof containing at least 85% acrylonitrile with an aqueous zinc chloride solution or aqueous mixed solution containing Zinc chloride as a principal constituent and water soluble metallic chloride or mixable organic substance, as a solvent for acrylonitrile polymers.

It has been known from prior publications such as patents to Rein, Stanton and others, that the shaped articles from polyacrylonitrile or acrylonitrile copolymer thereof may be obtained by the Wet spinning process with a solvent for :acrylonitrile polymer, such as aqueous Zinc chloride solution or aqueous mixed salt solution, which contains Zinc chloride as a principal constituent and has a specific range of chemical composition and concentration. However, it has frequently been described that when acrylonitrile fiber is prepared by using an aqueous zinc chloride solution as a solvent for aorylonitrile polymer, the resulting fibers tend to be hard and of a poor whiteness. For instance, Shodo Katayama has pointed out, on pp. 198-199 and p. 203 in his Work Acrylic Fibers (published in May 1959), that the acrylic fibers which are manufactured according to the invention by Stanton et a1. indicate the defects referred to the above in the case of an aqueous zinc chloride solution being used as a solvent, and moreover, that the desalting of spun tows is diflicult. The present inventors met with the disadvantages referred to the above in their preliminary investigations. However, it has been discovcred from our subsequent exhaustive investigations that the above defects are not substantial and if previous in vestigations by other investigators had been directed to the desalting of spun tows, such defects would not have occurred. Namely, it has been ascertained that the aforementioned defects can be avoided entirely, provided that :a complete desaltingis performed for spun gel tows, With the result that excel-lent fibers can be obtained which are free from the above defects and are, soft, resilient and resistant to coloring to yellow at high temperatures and have excellent physico-chemical properties.

If the desalting of spun tows is insufiicient and the spun tows contain zinc chloride, still above 1% by weight, the resulting acrylic fibers often become hard and tend to be discolored on heating. On the other hand, when the fibers contain Zinc chloride below about 1%, preferably below about 0.5% and especially below 0.1%, the defects referred to above are substantially negligible or are eliminated completely.

atent "ice However, when an aqueous solution containing zinc chloride as the principal constituent is used as the solvent for spinning solution of acrylonitrile polymer, there has been a substantial difficulty in the industrial production of excellent acrylic fibers containing negligible amount of Zinc chloride and hitherto there has been no satisfactory desalting process. On an experimental scale, it has been possible to rinse the spun tows or staple in hot water for several days, in order to remove zinc chloride from them, but such a treatment is not ap' plicable economically for industrial production. It has been almost impossible to lower the content of zinc chloride below 0.5% by washing the spun gel-tow in Water, even though performed in an extended period of time. In United States Patents 2,648,592 and 2,648,593 Stanton has proposed the methods of removing zinc chloride from the acrylic fiber, by immersing the tows, before it had been dried, in an aqueous solution of water soluble alkalimetal salt of ethylene diamine tetraacetic acid, or of poly-phosphoric acid :as the sequestering agent for the zinc ion in the coagulated tows. However, not only the cost of the sequestering agent is high in this process, but the desalting effect, as seen in an example described in such patent specifications, is rather incomplete as compared with the :desalting effect by this invention, as described later.

The present invention provides an extremely simple and practical process whereby the synthetic acrylonitrile fibers containing a negligible amount of zinc chloride are produced by using an aqueous zinc chloride solution or aqueous solution containing zinc chloride as the principal constituent as the solvents for acrylonitrile polymers. The present invention is accomplished by acidifying the baths of water for Washing the gel-tow drawn up from a coagulating bath wherein the pH value of said water baths are held below 3 and the bath temperatures kept in the range from room temperature to about C. It is, of course, within the scope of this invention to add in advance a suitable amount of acid into the spinning solution prior to said dope being transferred to la coagulating operation, i.e., such a pretreatment may be effective on shortening the lentgh of washing baths of pH below 3. However, the practically effective desalting of gel-tow cannot be attained by such a pretreatment alone, without acidifying the subsequent water baths for washing, as shown in Example 1.

To attain the same effect, it is also within the scope of this invention that the tows drawn up from a coagulating bath are treated in an acid bath before entering into the acidified Washing bath. This acid bath contains aqueous solution of considerable amount of acid, i.e., its acid concentration being from 1 N to 12 N, and it is pro vided between the coagulation bath and washing bath and held in the range from room temperature to about 70 C.

According to the process by this invention, the desalting of the gel-formed tows can be carried out quickly and effectively. Either inorganic acid or organic acid may be used. However, it is not preferable to use such an acid as combined with zinc ions in an aqueous medium to form an insoluble zinc compound. Accordingly, acidwater bath is defined as a bath in which the acid does not combine with Zinc ions in an aqueous medium to form an insoluble zinc compound.

The required conditions for the use of acids are as follows: Acid must be added to the water baths for washing the gel-tow drawn up from the coagulating bath so as to adjust the pH of value of the water below 3, because the desalting effect is considerably decreased when the pH is above 3. When strong acids such as hy- In the production of acrylic fibers by the wet spinning method with as aqueous solution containing mainly zinc chloride as the solvent for acrylonitrile polymers, it is technically evident that it is extremely effective for removing zinc chloride almost completely from spun geltow to pass the gel-formed tows through an aqueous medium below pH 3. When gel-formed spun tows are passed through the water-hath, zinc chloride held in the tows is gradually diffused out and simultaneously water enters inside of them. In the water-bath without acid the diffusion-substitution referred to above may also proceed until the residual amount of a zinc chloride in the gel tow will approach a degree of few percent by weight. However, when the residual amount of zinc chloride becomes below a percent by weight, the salt is hydrolyzed to form insoluble basic zinc chloride, zinc hydroxide, or hydrous zinc chloride all of which are precipitated in the gel-formed tows and can not be washed out.

The present inventors have discovered that the difficulties in desalting with water has been in fact due to the phenomenon described above. In this case, if the gelformed tows are washed in an aqueous medium admixed with acid, the acid will penetrate into the gel-formed tows to prevent the hydrolysis of zinc chloride contained in them, thereby facilitating the diffusion of zinc chloride out of the tows.

In order to attain the desalting purpose effectively by the present invention, it has been ascertained that the temperature of the washing medium containing acid is required to be held in the range of room temperature to about 70 C. The gel-tows formed in a coagulating bath enter into an acid bath or a water-bath of pH below 3. In this case, when the temperature of such acid bath or the water bath is below room temperature, the velocity of the diffusion of zinc chloride contained in the gel-formed tows into the water-bath will become slow and the desalting rate will consequently decrease. Therefore, it is preferable to keep the temperature of the acid bath following the coagulating bath or the water-bath of low pH between 70 C. and room temperature. When the temperature of the coagulating bath or the waterbath becomes above 70 C., the gel-tows become rapidly coagulated. In this case it has been ascertained by the present inventors that the absorption of acid into tows as well as the diffusion of the Zinc salt contained in the tows into the water-bath are remarkably obstructed, since the tows are coated with a dense skin of an extremely low diffusion constant. Namely, it is necessary to carry out the desalting operation before the perfectly dense solidification of spun tows, by controlling not only the tempenature and salt-concentration of the coagulating bath but also both the temperture and total length of the acid baths or water baths with a pH of less than 3. It is preferable, therefore, to keep the temperature of the coagulating bath below 40 C. and to pass the gel-tows through the subsequent water-baths with a pH of less than 3, before the dense solidification is completed.

According to the present invention, it can be readily and industrially [attained to decrease the weight percent of the zinc compound remained in :dry fibers to less than 0.05, converting it into the weight of zinc chloride. Moreover, no harmful result has been found .for the desalting operation mentioned above. The invention such as described in Japanese Patent No. 196,508 is entirely different from the process by the present invention both in the inventive idea and technical content, since the former is limited with the object of improving the stability of acrylonitrile polymers against thermal discoloration, where the polymers are tneated with an aqueous solvent held at to 150 C. and of pH less than 5.8 by addition of a suitable amount of acid. In that patent, there is no description with respect to the case of zinc chloride solvent for the polymers. Furthermore, in the optimum range of temperature of 85 to 140 C. for that invention, no significant desalting eifect has been observed, because at these temperatures the immersed tows have a dense coagulated skin which hinder the penetration of acid into the tows as well as the diffusion of Zinc chloride out of them.

Japanese Patent No. 232,744 describes that a mixed aqueous solution of zinc chloride and hydrochloric acid is usable as a solvent for acrylonitrile polymers and mouldings whereby the little amount of remaining zinc chloride can be obtained by coagulating the acrylonitrile polymer solution made with the said mixed solvent. However, no definite description is given in said patent in reference to the desalting effect, and as shown in the table of Example 1, any effective desalting cannot be attained by acidifying only the spinning dope in the coagulating bath.

On the contrary, as described in the present invention the desalting is performed in contact with an aqueous solvent with pH less than 3 before the complete solidification of gel-like fibrous mouldings is attained. Not only the operation conditions are different from those of the Japanese Patent No. 232,744, but also the desalting effect is particularly remarkable as illustrated in the following examples. Thus, the present invention is quite distinguishable from those described in the above patents, in its inventive idea and industrial effects.

Example 1 A spinning solution comprising 90 parts of saline solvent containing 54% Zinc chloride, 4% sodium chloride and 42% water and 10 parts of acrylonitrile copolymer having a molecular weight of 80,000, and composed of 92% acrylonitrile and 8% methyl acrylate was prepared by a solution-polymerization process. The spinning solution was spun out through 5,000 hole nozzles into a co- :agul'ating bath of an aqueous salt solution prepared by diluting the saline solvent to 15% concentration.

The gel-tows from the coagulating bath were then passed successively through the 1st, 2nd, and 3rd waterbaths, each of them being 6 m. long and then dried, stretched and relaxed, in the following to give the fibers necessary properties.

Hyhdrochloric acid was added to the coagulating bath and water-bath.

The results on desalting effect were indicated in the following table:

Zinc Coagu- First Second Third chloride latiug waterwaterwaterin dry bath bath bath bath fiber,

percent Exp.:

A Temp, C 25 25 40 55 Example 2 Experiment was carried out under the same conditions as described in Example 1, except that sulfuric acid was used as an additional acid instead of hydrochloric acid.

The results of Example 2 were as follows: the experiments A, B, C correspond to those in Example 1.

Residual ZnCl in dry fiber, percent Example 3 A bath of aqueous hydrochloric acid solution of 20% concentration (3 m. long) was provided between the coagulating bath and the 1st water-bath, in Experiment D of Example 1.

The gel tows passed through the acid bath entered into the 1st water-bath, accompanied with a considerable amount of hydrochloric acid. Therefore, the first waterbath showed a pH of about 2.5 in continuous operation. The residual zinc chloride of resultant fibers could be decreased to 0.25%

Example 4 R d 1 z 01. 1 The results were as below: esl m n n dry fiber, percent What we claim:

1. A method for preparing spun gel-tow fibers by a Wet spinning process from a spinning solution containing acrylic co-opolymers including at least 85% acry-lonitrile employing aqueous saline solvents containing zinc chloride, the improvement which comprises Washing the spun gel-tows in an acidwater bath at a pH of not greater than 3 while controlling and maintaining the temperature below C. thereby facilitating removal of zinc chloride from the spun gel-tows.

2. A method according to claim 1 wherein an acid treatment is provided, whose acid concentration is held in the range from 1 normal to 12 normal and the reaction temperature is maintained between 15 and 70 C.

3. A method for preparing spun gel-tow fibers by a wet spinning process from a spinning solution containing acrylic co-polymers including at least acrylonitrile employing aqueous saline solvents containing a mixture of water soluble metallic chlorides including zinc chloride, the improvement which comprises washing the spun geltows in an acid-water bath of a pH of not greater than 3 while controlling and maintaining the temperature below 70 C. thereby facilitating removal of zinc chloride from the spun gel-tows.

References Cited in the file of this patent UNITED STATES PATENTS 2,431,956 Moody Dec. 2, 1947 FOREIGN PATENTS 636,476 Great Britain May 3, 1950 

3. A METHOD FOR PREPARING SPUN GEL-TOW FIBERS BY A WET SPINNING PROCESS FOR A SPINNING SOLUTION CONTAINING ACRYLIC CO-POLYMERS INCLUDING AT LEAST 85% ACRYLONITRILE COMPLOYING AQUEOUS SALINE SOLVENTS CONTAINING A MIXTURE OF WATER SOLUBLE METALLIC CHLORIDES INCLUDING ZINC CHLORIDE, THE IMPROVEMENT WHICH COMPRISES WASHING THE SPUN GELTOWS IN AN ACIDWATER BATH OF A PH OF NOT GREATER THAN 3 WHILE CONTROLLING AND MAINTAINING THE TEMPERATURE BELOW 70*C. THEREBY FACILITATING REMOVAL OF ZINC CHLORIDE FROM THE SPUN GEL-TOWS. 